There has been conventionally known a light-extinction-type smoke sensor that senses smoke based on an extinction rate of detection light, as one mode of a smoke sensor that senses smoke generated by fire or the like. Among these light-extinction-type smoke sensors, as a sensor capable of monitoring a relatively wide area, there is a photoelectric separated smoke sensor.
FIG. 18 is a system configuration diagram of a conventional photoelectric separated smoke sensor. The photoelectric separated smoke sensor 100 includes light transmitting units 101A to 101C that transmit detection light, and light receiving units 102A to 102C that receive the detection light, by oppositely laying out these units with a distance sandwiching a monitoring area between these units. The light receiving units 102A to 102C laid out oppositely to the light transmitting units 01A to 101C, respectively receive the detection light transmitted from the light transmitting units 101A to 101C, respectively. The light receiving units 102A to 102C calculate a light extinction amount and a light extinction rate of the detection light, respectively. When the light extinction amount becomes equal to or larger than a predetermined reference value, the light receiving units 102A to 102C determine that smoke is generated (fire occurs), and output accident-warning audible signals indicating the occurrence of fire, to a receiving device 104 connected by wire through a control line 103.
To properly determine the light extinction amount of the detection light, the transmission timing of the detection light transmitted by the light transmitting units 101A to 101C needs to be synchronized with the reception timing of the detection light received by the light receiving units 102A to 102C. For this purpose, conventionally, the light transmitting units 101A to 101C and the light receiving units 102A to 102C are connected to each other by wire using the control line 105. The light transmitting units 101A to 101C output control signals (synchronization signals) to the light receiving units 102A to 102C via the control line 105. The light receiving units 102A to 102C receive the light at a predetermined interval based on a synchronization timing specified by the synchronization signals, thereby establishing the synchronization between the light transmitting units 101A to 101C and the light receiving units 102A to 102C (for example, see Patent Document 1).
Patent Document 1: Japanese Patent Application Laid-open No. H08-227489